This invention relates to improved composite articles subjected to noncontinuous loads which result in stress concentrations due to points of geometrical change within the article. More particularly, this invention relates to an improved composite sports racket for tennis, badminton, squash or the like, and to a method for fabricating the same.
Sports rackets used to strike or hit a ball or shuttlecock are typically composed of a handle and an open-faced racket head. The racket head is disposed at the upper part of the racket, generally in a form having an elliptical or oval shaped cross section. A network of stringing is strung in tension across the face of the racket head. Tensions creating forces in the string on the order of 45 to 65 pounds are said to produce excellent playing characteristics in tennis. It is important that the string stresses be evenly distributed throughout the structure to prolong string life and improve off center hits.
The strings are normally threaded through apertures in the head frame that are provided to accommodate the stringing. The apertures may be molded, punched, drilled or otherwise formed in the frame. The apertures represent abrupt changes in the geometry of the frame which inherently cause concentrations of stress and result in a substantial loss of strength in an area subjected to stress from both the static forces of string tension and the non-continuous dynamic forces resulting from impact and return of the balls. Hence, it is important to design the apertures so as to prevent localized deformation and string wear.
Fiber reinforced plastic or composite materials have been used to produce sports rackets with satisfactory playing characteristics. The high mechanical strengths and anisotropic characteristics of composites make these readily adaptable to withstanding the major bending and twisting forces to which sports rackets are subjected. The composite racket frames, moreover, are often formed in a one component construction obviating disadvantages of wood or metallic constructions that may variously use multiple pieces, rivets, welds or laminations which can loosen or break. Fabrication of stringholes in composite materials, however, will necessarily sever some of the fibers in the plastic matrix and result in fiber discontinuities at the stringholes. Because of these fiber discontinuities cracks, once developed, can readily propagate from stringhole to stringholes resulting in an overall decrease in the structural strength of frame and, consequently, in a decrease in durability.
Thus, there exists a need for a composite sports racket which retards the initiation and propagation of cracks in the area of the stringholes.